BACKGROUND: The appearance of an exceptional number of cases of Brucellosis at the end of 1996 in workers at a slaughterhouse led us to suspect an epidemic outbreak among this group. This study shows the methodology followed in the analysis of this outbreak as well as the results obtained. METHODS: 1.- Epidemiological description of the outbreak: number of animals with brucellosis slaughtered, collection of information from different sources on the number of those affected: from the mutual insurance company, the record of working days lost, an epidemiological monitoring system and a survey amongst the workers. 2.- A case and control study was designed in order to determine, firstly, non-occupational risks - ingestion of fresh cheese or milk and care of animals - and secondly, occupational risks, depending on the job normally undertaken. 3.- To verify this a retrospective cohort study was designed. The exposed group was made up by workers in the slaughter area and the unexposed group comprised the remainder; any worker giving a positive result to the Rose of Bengal test and IgM brucellosis antibodies in serum was considered as a case. RESULTS: The description of the outbreak enabled us to establish that the cases occurred at the moment when most animals were slaughtered, that only occupational risks were relevant, that there were more symptomatic cases than notified ones, and that the slaughter line operators showed higher rates of attack than the remainder of the workers. CONCLUSIONS: This study analyses the possible causal relationship between analyzed exposure and the appearance of cases of brucellosis in workers - a fact which backs the existing scientific evidence on the importance of the respiratory tract as a mechanism of transmission of this disease in the workplace.